As advances in science increasingly demand the measurement of chemical species or reactions in microscale domains, fundamental and applied studies on various types of ultramicroelectrodes (UMEs) have attracted considerable attention. Voltammetric UMEs (disks, rings, or conical tip electrodes of micro- or submicrometer dimension) have been widely used to study the release of neurotransmitters from a single cell, rapid homogeneous or heterogeneous electron-transfer kinetics, and electrochemical reactions in poorly conducting media and applied as tips in high-resolution scanning electrochemical microscopy (SECM). Potentiometric UMEs, comprising simple metal wires, metal wires coated with insoluble salt (e.g., Ag/AgCl), or conventional ion-selective electrodes (ISEs) fabricated in micropipets, have also been developed to investigate the in vivo ionic activity of single cells and, more recently, as a probe of SECM. However, the application of potentiometric UMEs for SECM studies is rather limited in practice, in part due to the complexity of constructing such system.
Several types of potentiometric UMEs are found in the literature. Ag/AgCl microelectrode disks (10- and 50-μm diameter) were used to probe the diffusion of Ag+ on a planar Ag electrode and to measure ion flux of Cl− over polyaniline films electrodeposited on Pt. Antimony-based pH microdisk electrodes (tip size, ˜3-μm diameter) were also fabricated and used to image the local pH changes in several model chemical systems, e.g., reduction of water on Pt electrode, the corrosion of AgI in aqueous potassium cyanide, enzyme reactions of immobilized urease, and metabolic activity of yeast cells. Neutral carrier-based micropipet ISEs (typical tip diameter, 1-20 μm) were used as probes in SECM to image local concentration profiles of NH4+, K+, and Zn2+ ions.
Previous studies suggest that the fabrication of potentiometric UMEs requires a substantial effort in preparing submicrometer-sized electrode tips or in the construction of micropipet-based ion-selective electrodes. Ion-selective metal oxide layers (e.g., iridium oxide, silver/silver chloride, etc.) may be deposited at the exposed end of the submicrometer-sized electrode tip or the tip coated with ionophore-doped solvent polymeric membranes to result in a desired ion selectivity. However, the affixed layer on a small electrode area may not be stable even for a short period of time. Therefore, despite some advances there still remains the need for improved electrode designs.